单选题 What was the origin of the oil which now drives our motorcars and aircraft? Scientists are 21 about the 22 of coal, but they do not seem so 23 when asked about oil. They think that the oil 24 the surface of the earth originated in the 25 past, and was formed 26 living things in the sea. Countless billions of 27 sea creatures and plants lived and 28 to the sea bed. They were covered with huge 29 of mud; "and by 30 of chemistry, pressure and temperature were changed through long 31 into what we know as oil. For these creatures to become oil, it was necessary that they should be 32 between layers of rock for an enormous 33 of time. The 34 that oil originated in the sea is 35 by a glance at a map 36 the chief oil fields of the world; very 37 of them are far distant from the oceans of today. In some places gas and oil 38 to the surface of the sea from its bed. The rocks in which oil is found are 39 marine origin too. They are sedimentary rocks, rocks which were 40 by the action of water on the ocean.